A conventional projector is mainly used for presentations at, for example, a conference, and therefore there is little need for the projector main body to have a speaker. This is because the presenter usually explains the data projected on a screen using a microphone. As such, the speech is audible as audio information in a conference hall or the like through speakers.
However, as DVD system use spreads in the future, users will probably prefer larger screens. When a projector is used as a large screen image device, an acoustic system is necessary. In certain use environments, however, there is hardly space to provide a stereo surround system or a 5.1 channel surround system. Further, such a sound system is costly. As such, the value of a projector as a tool for providing a home theater or for showing educational materials is almost nonexistent.
A speaker that allows the projector to be used as if it is a radio-cassette player could be installed in the projector main body. A general loudspeaker, however, is insufficient in volume and cannot provide enough acoustic sound if installed in the projector. That is, if a speaker is provided in the projector main body and sound is generated from it, the positional difference between the sound source and the screen that actually shows images inevitably makes the viewers feel that the sound and the action on the screen are not exactly synchronized.
The idea of storing an ultrasonic speaker using an ultrasonic transducer in a projector has been suggested. Such a conventional ultrasonic speaker is shown in FIG. 8. The ultrasonic speaker includes an audio frequency wave oscillating source 81 that generates a signal in the audio frequency band, a carrier wave oscillating source 82 that generates a carrier wave, a modulator 83, a power amplifier 84, and an ultrasonic transducer 85.
In the above conventional ultrasonic speaker, a carrier wave in the ultrasonic frequency band output from the carrier wave oscillating source 82 is modulated, with a signal output from the audio frequency wave oscillating source 81 by the modulator 83. The modulated signal amplified by the power amplifier 84 drives the ultrasonic transducer 85. In this manner, the modulated signal is converted into a sound wave in a finite amplitude level by the ultrasonic transducer 85, and the sound wave is radiated into a medium (air), so that the non-linear effect of the medium (air) allows signal sound in the original audio frequency band to be reproduced.
In this case, the range of reproducing the signal in the audio frequency band corresponds to a beam-shaped range in the direction of the radiation axis from the ultrasonic transducer 85.
The construction of such a conventional ultrasonic transducer used in an ultrasonic speaker is shown in FIG. 9. The conventional ultrasonic transducer is mostly a resonant type, using piezoelectric ceramic as an oscillating element. The ultrasonic transducer shown in FIG. 9 uses piezoelectric ceramic as the oscillating element to convert an electric signal into an ultrasonic wave and vice versa (transmits/receives an ultrasonic wave). The bimorph ultrasonic transducer shown in FIG. 9(A) includes two pieces of piezoelectric ceramic 91 and 92, a cone 93, a case 94, leads 95 and 96, and a screen 97.
The pieces of piezoelectric ceramic 91 and 92 are joined with each other and the leads 95 and 96 are connected to the surfaces opposing the joining surface.
A unimorph ultrasonic transducer, shown in FIG. 9(B), includes a piece of piezoelectric ceramic 101, a case 102, leads 103 and 104, an internal interconnection 105, and a glass part 106. The piece of piezoelectric ceramic 101 is connected with the lead 103 through the internal interconnection 105 and grounded to the case 102.
The resonant ultrasonic transducer takes advantage of the resonance of the piezoelectric ceramic, and therefore the transmitting/receiving characteristics for the ultrasonic waves are good only in several relatively narrow frequency bands around the resonance frequency. Therefore, the quality of the generated sound is not high.